1. Field of the Invention
Although the nitric phosphate technologies described, supra, eliminate the problem of phosphogypsum production, they are generally incapable of producing high-analysis phosphatic fertilizers comparable in phosphate grade to TSP, DAP, and MAP, which are now widely accepted by American farmers and extensively used in bulk blending operations. The relatively low phosphate content of nitric phosphates results from the fact that some calcium is left in the products and part of the nitrogen is present in the bulky nitrate form.
In searching for a procedure to produce high-analysis phosphatic fertilizer by direct acidulation of phosphate rock with nitric acid, it is tempting to treat the rock phosphate with only enough acid to convert the phosphate component to agronomically available, but water insoluble, dicalcium phosphate and then separate the soluble calcium nitrate by simply leaching the mixture with water. As indicated by the following equations, dicalcium phosphate might be produced at one-half the acid consumption required for the production of monocalcium phosphate (the chief component of TSP). EQU 6 HNO.sub.3 +Ca.sub.10 (PO.sub.4).sub.6 F.sub.2 .fwdarw.6 CaHPO.sub.4 +3 Ca(NO.sub.3).sub.2 +CaF.sub.2 EQU 12 HNO.sub.3 +Ca.sub.10 (PO.sub.4).sub.6 F.sub.2 .fwdarw.3 Ca(H.sub.2 PO.sub.4).sub.2 +6 Ca(NO.sub.3).sub.2 +CaF.sub.2
In practice, however, the second reaction, supra, proceeds quite readily while the first reaction is generally not observed (U.S. Pat. No. 4,113,842, NcCullough, et al., Sept. 12, 1978, assigned to the assignee of the present invention). When phosphate rock is acidulated with a quantity of acid sufficient only for the production of dicalcium phosphate, the acid is found to preferentially react to produce monocalcium phosphate and leave the remainder of the rock unacidulated. This phenomena results from the presence of low (&lt;0.05%) concentrations of uncomplexed fluoride ion in the mix and is explicable in terms of the phase diagram for the system CaO--P.sub.2 O.sub.5 --HF--H.sub.2 O [T. D. Farr, G. Tarbutton, and H. T. Lewis, J. Phys. Chem. 66, 318, 1962], which shows no region of stability for CaHPO.sub.4. The invariant point for the solid phases Ca(H.sub.2 PO.sub.4).sub.2.H.sub.2 O, Ca.sub.10 (PO.sub.4).sub.6 F.sub.2, and CaF.sub.2 occurs at a pH of about 0.7. Monocalcium phosphate and calcium fluoride precipitate at pH values below this value, while fluoroapatite and calcium fluoride precipitate at pH values above this value.
Fertilizer-grade dicalcium phosphate can be produced by addition of lime to merchant-grade (54% P.sub.2 O.sub.5) phosphoric acid, from which considerable fluoride has been volatilized during concentration, provided enough impurity silica and aluminum are present (as is generally the case) to complex the remaining fluoride. This procedure, however, consumes about three times more acid than the stoichiometric quantity for direct acidulation. In addition, an input of lime is required. Hence, the production of fertilizer-grade dicalcium phosphate by this procedure is economically unattractive relative to the production of TSP, DAP, and MAP, which may be obtained with similar acid consumptions.
Even though dicalcium phosphate is not water soluble, it is recognized as a highly effective, completely available phosphatic fertilizer (Superphosphate, Its History, Chemistry, and Manufacture, Agricultural Research Service, U.S. Department of Agriculture, Washington, D.C., Issued December 1964, p 172). Although the debate over the relative merits of water soluble versus citrate soluble (available) fertilizers has continued for many years, it is now generally recognized that aqueous phosphate is highly reactive and, upon application to soil, quickly reverts to water insoluble forms by reaction with soil minerals. This process is referred to as "phosphate fixation" and generally results in phosphorus being an almost completely immobile plant nutrient, as opposed to nitrate which remains quite soluble in soil solution and may be leached to considerable depths below the point of application (E. C. Sample, R. J. Soper, and G. J. Racz, "Reactions of Phosphate Fertilizers in Soils," The Role of Phosphorus in Agriculture, ed. by F. E. Khasawneh, E. C. Sample, and E. J. Kamprath, published by American Society of Agronomy, Crop Science Society of America, Soil Science Society of America, Madison, Wis., 1980; "Water-Solubility, Safeguard or Sacred Cow?" Phosphorus & Potassium 160, 23-32, March-April, 1989).
The nature of the P fixation processes depend primarily upon the soil type. In the generally calcareous solis of the temperate zones the fixation process may be governed primarily by the equilibria within the calcium phosphate family of compounds--soluble phosphate being converted rapidly to dicalcium phosphate which is then slowly (due to its low solubility) converted to even more insoluble calcium phosphate salts, such as octacalcium phosphate, apatite, etc. In the generally acidic (calcium deficient) soils of the tropics and subtropics, water soluble phosphates may be almost instantaneously fixed as highly insoluble iron or aluminum phosphates.
The type of phosphate fixation may govern to some extent the plant response which is obtained when water soluble versus simply "available" phosphatic fertilizers are compared in soils that are deficient in phosphate. Hence, simply "available" phosphates often give better crop response when applied to the phosphate deficient acid soils of the tropics and subtropics, while water soluble phosphates often given better crop response when applied to phosphate deficient calcareous soils of the temperature zones. However, it should be noted that these results are strongly influenced by other factors such as the type of crop, the state of subdivision (granule size) of the fertilizer, placement of the fertilizer, and the presence of other nutrients.
It is emphasized that the above generalizations apply only to soils that are deficient in phosphorus. It must be realized that the buildup of residues from phosphate fertilization is a common feature of modern agriculture in developed countries. Indeed, there are vast areas of farmland within the United States in which crops show no response to applied phosphate fertilizer, regardless of its form (G. L. Terman, "Phosphate Sources: Agronomic Effectiveness in Relation to Chemical and Physical Properties," Proceedings of the Fertilizer Society, No. 123, 1971). It is further recognized that on the average, less than 20 percent of the phosphate applied to the soil during the year can be accounted for in the same years crop. Under such circumstances, the only apparent rationale for the application of phosphate fertilizer appears to be for maintenance purposes. There is no evidence to indicate that water soluble phosphatic fertilizers are more beneficial for this purpose than are simply available phosphates. In fact, some studies indicate that non-water soluble but citrate soluble materials, such as dicalcium phosphate, may react with the soil more slowly and so retain their individual availability longer (N. J. Barrow, "Evaluation and Utilization of Residual Phosphorus in Soils," The Role of Phosphorus in Agriculture, ed. by F. E. Khasawneh, E. C. Sample, and E. J. Kamprath, published by American Society of Agronomy, Crop Science Society of America, Soil Science Society of America, Madison, Wis., 1980).
2. Description of the Prior Art
Early recognition of the commercial possibility of producing dicalcium phosphate by pyrolysis of mixtures of soluble phosphate and calcium nitrate is taught in U.S. Pat. No. 2,134,013, Turrentine, Oct. 25, 1938. Turrentine reacted phosphate rock with volatile inorganic acids, such as hydrochloric acid, nitric acid, or oxides of nitrogen, in amounts sufficient to convert the contained phosphate to a water soluble form, and then heated the mixture to drive off a portion of the reacted acid and leave a product containing primarily calcium nitrate and dicalcium phosphate.
In the practice of the teachings of U.S. Pat. No. 2,211,918, Turrentine, Aug. 20, 1940, there is employed a similar procedure to produce fertilizers containing double salts of dicalcium phosphate with calcium nitrate or calcium chloride. These products were claimed to be less hygroscopic and to contain more water soluble phosphate than simple mixtures of the calcium nitrate or chloride with dicalcium phosphate.
During the height of the sulfur shortage of the early 1950s, Turrentine expounded the use of this technology to prevent the then perceived rapid depletion of brimstone reserves (Chemical and Engineering News 29, (34), 3454-56, Aug. 20, 1951). However, it should be noted that in each case the proposed products were mixed N-P or Cl-P materials, no provision being made for the separation of the phosphate from the nitrate or chloride salts.
In Austrian Patent No. 176,219, Joham, Sept. 25, 1953 there is described the reaction of nitric acid with phosphate rock to produce a liquid reaction mixture which is subsequently evaporated and dried at 200.degree. C. to provide a solid material which was dispersed in water and filtered to produce a concentrated calcium nitrate solution and a solid product containing 34.3 percent P.sub.2 O.sub.5, of which 88 percent was citrate soluble. A variation of the procedure involved the addition of lime to the initially produced acidulate to bring the pH to 1.4 and to precipitate CaF.sub.2, which was then filtered along with the gangue. Subsequent treatment of the filtrate, as described above, resulted in a solid product containing 44.8 percent P.sub.2 O.sub.5, of which 89 percent was citrate soluble. It should be noted, however, that this procedure probably also results in partial reversion of phosphate to fluorapatite as indicated by the phase equilibria of Farr, et al., supra. Furthermore, neither variation of the procedure provides for a completely citrate soluble phosphate product, or for the recovery of the excess nitric acid liberated during the evaporation process.
U.S. Pat. No. 2,753,252, Barnes, July 3, 1956, it is proposed that a substantially completely citrate soluble phosphatic product may be prepared by reacting phosphate rock with nitric acid, filtering the gangue, evaporating free water from the acidulate by boiling, heating the resulting residue to about 180.degree. C. to about 190.degree. C. to expel all water of hydration, dispersing the residue in anhydrous ammonia to dissolve the calcium nitrate, and finally filtering the mixture to obtain the phosphate product and a filtrate containing anhydrous ammonia and calcium nitrate. The filtrate is further processed with water and carbon dioxide to produce an ammonium nitrate solution and by-product calcium carbonate. Barnes claims a 95.8 percent recovery of the P.sub.2 O.sub.5 initially present in the rock, but makes no provision for the recovery of nitric acid liberated during the heating processes.